This invention relates to investigation of earth formations and, more particularly, to a method and apparatus for determining properties of earth formations using sonic well logging which can characterize earth formations exhibiting complex acoustic behavior, such as can occur in anisotropic and/or inhomogeneous formations.
It is well known that mechanical disturbances can be used to establish acoustic waves in earth formations surrounding a borehole, and the properties of these waves can be measured to obtain important information about the formations through which the waves have propagated. Parameters of compressional, shear and Stoneley waves, such as their velocity (or its reciprocal, slowness) in the formation and in the borehole, can be indicators of formation characteristics that help in evaluation of the location and/or producibility of hydrocarbon resources.
An example of a logging device that has been used to obtain and analyze sonic logging measurements of formations surrounding an earth borehole is called a Dipole Shear Sonic Imager (xe2x80x9cDSIxe2x80x9dxe2x80x94trademark of Schlumberger), and is of the general type described in Harrison et al., xe2x80x9cAcquisition and Analysis of Sonic Waveforms From a Borehole Monopole And Dipole Source For The Determination Of Compressional And Shear Speeds And Their Relation To Rock Mechanical Properties And Surface Seismic Dataxe2x80x9d, Society of Petroleum Engineers, SPE 20557, 1990. In conventional use of the DSI logging tool, one can present compressional slowness, xcex94tc, shear slowness, xcex94ts, and Stoneley slowness, xcex94tst, each as a function of depth, z. [Slowness is the reciprocal of velocity and corresponds to the interval transit time typically measured by sonic logging tools.] Typically, the subsurface formations are considered to be homogeneous and isotropic material, where the compressional and shear velocities, Vc and Vs, of the formations are only a function of depth.
It is known that formations can be anisotropic, where the compressional and shear slownesses are a function of azimuth, xcex8. Anisotropy can occur, for example because of layered shales, aligned fractures or differences in the magnitudes of the principal stresses in the formations.
It is also known that formations may be inhomogeneous, where the slownesses become a function of radial distance r, from the borehole. Inhomogeneity can be caused, for example, by mud-shale interactions or by mechanical damage due to stress concentrations.
It is among the objectives of the present invention to provide an improved technique for characterizing earth formations exhibiting complex acoustic behavior.
A form of the present invention provides a technique for jointly processing monopole and dipole measurements from a sonic logging device to obtain a more complete characterization of the acoustic behavior of formations surrounding an earth borehole. From an integrated sonic inversion, improved estimates of slowness, as well as characterization of the near wellbore damage region, can be obtained.
In accordance with a form of the invention, a method is set forth for determining properties of an earth formation surrounding an earth borehole, comprising the following steps: (a) providing a logging device that is moveable through the borehole; (b) transmitting sonic energy into the formation, receiving, at the logging device, sonic energy that has travelled through the formation, and producing signals representative of the received sonic energy; (c) determining, from the signals, whether the formation is anisotropic; (d) determining, from the signals, whether said formation is inhomogeneous; and (e) responsive to the determinations of steps (c) and (d), outputting a characterization of the formation as one of the following types: isotropic/homogeneous, anisotropic/homogeneous, isotropic/inhomogeneous, and anisotropic/inhomogeneous. In an embodiment of this form of the invention, the step (b) includes transmitting sonic energy from a monopole source, and receiving sonic energy from the monopole source at a plurality of different transmitter-to-receiver spacings on the logging device, and the step (d) includes determining whether said formation is inhomogeneous from deviations between signals at different transmitter-to-receiver spacings. In this embodiment, the step (b) also includes transmitting dipole shear sonic energy, receiving dipole shear sonic energy that has travelled through the formation, and producing signals representative of the received sonic energy over a range of frequencies, and the step (c) includes determining whether the formation is anisotropic from said signals. It will be understood that the step of transmitting dipole shear sonic energy can comprise producing what is commonly referred to as a flexural wave by employing a dipole source in the borehole to cause a flexing of the borehole wall.
In a further form of the invention, a method is set forth for determining properties of an earth formation surrounding an earth borehole, comprising the following steps: (a) providing a logging device that is moveable through the borehole; (b) transmitting monopole and dipole sonic energy from the logging device into the formations and receiving, at the logging device, monopole and dipole sonic energy that has travelled through the formations, and producing measurement signals representative of the received monopole and dipole sonic energy; (c) devising a plurality of formation models of different complexities; (d) comparing model signals from the models with the measurement signals; and (e) selecting one of the models based on said comparing step. In an embodiment of this form of the invention, the plurality of models includes four models, the models, in order of increasing complexity, being a homogeneous/isotropic model, a homogeneous/anisotropic model, an inhomogeneous/isotropic model, and an inhomogeneous/anisotropic model. Also in this embodiment, the step (e) selection of a model takes into account model complexity as well as the results of the comparing step.
A feature of the new sonic characterization of a form of the invention is that a joint processing of compressional and shear data is being used to indicate the xe2x80x9cstatexe2x80x9d of the formation, in the context of inhomogeneity and/or anisotropy. This xe2x80x9cstatexe2x80x9d of the formation would be a function of depth. Some of the applications fall into two general categories; those requiring xe2x80x9cundamagedxe2x80x9d parameters, and those applications requiring xe2x80x9cdamagedxe2x80x9d parameters.
xe2x80x9cUndamagedxe2x80x9d parameters (the velocities Vp and Vs of the virgin formation) can be used by geophysics and petrophysics experts to evaluate the reservoir and overburden rock in the traditional manner. With the method hereof, there will be improved confidence that the measurements have a deep enough depth of investigation to be representative of the undamaged formation.
Near wellbore xe2x80x9cdamagedxe2x80x9d parameters are new information of use to drilling and completions engineers. As an example, for inhomogeneity in a reservoir caused by stress concentrations, the location of damaged zones can influence the completion and perforating strategy. One can selectively perforate to avoid damaged zones. Also, detection of damage from sonics can have an impact on other logging measurements that have shallow depths of investigations, so suitable adjustments can be made.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.